The present invention relates to a projection optical system for use in a precise copy which is suitable for use in the case of ultrashort wavelengths such as ultraviolet rays, far ultraviolet rays, and the like.
Hitherto, a projection optical system for use in a precise copy which consists of a refractive system using lenses has been known as shown in, e.g., Japanese patent publication (JP-B) No. 12966/82. Another projection optical system consisting of a reflecting system has also been known as shown in Japanese laid-open patent publication (JP-A) No. 17196/80.
The conventional projection optical systems for use in a precise copy will be described hereinbelow with reference to the drawings.
As shown in FIG. 1, the optical system consisting of the refracting system uses a plurality of lenses consisting of optical glasses G and optical crystal materials C and is the reducing system having an image magnification of 1/10. On the other hand, as shown in FIG. 2, the projection optical system using the reflecting system has a telecentric constitution such that the light emitted from an object O are sequentially reflected by a main mirror 100, an auxiliary mirror 101, and the main mirror 100, thereby forming an image I of equal magnification of 1/1 onto the same plane as the object O.
A resolution limit D of the projection optical system is expressed using an equation of Rayleigh as follows. ##EQU1## where, NA is a numerical aperture on the side of the image of the optical system and .lambda. is a wavelength which is used. In order to improve the resolving power, it is sufficient to reduce the wavelength .lambda. or to increase the numerical aperture NA of the optical system. However, the improvement of the numerical aperture NA makes the optical design of the lenses considerably difficult. Therefore, the reduction of the wavelength .lambda. is becoming the main consideration.
In the conventional refracting optical system shown in FIG. 1, when the wavelength .lambda. is reduced to a value in the range of the ultraviolet rays, the transmittance extremely decreases. For example, even if the glasses whose characteristics were adjusted for use in the wavelengths in the range of, particularly, the ultraviolet rays, such as glasses of UBK7 made by schott, Co., Ltd. are used, when the wavelength is 280 nm, the transmittance extremely decreases to 23% (when the thickness of glass is 5 mm). On the other hand, among the optical crystal materials C, there are such materials that a transmittance of about 80% can be obtained even in the case of the wavelength of about 200 nm as in the synthetic crystal of TiF.sub.2, CaF.sub.2, or Kcl. However, in general, it is difficult to obtain a large sized crystal material and the working efficiency is very bad and it is difficult to work with a high precision. In addition, since the number of kinds of such crystal materials is limited, the degree of design freedom is small. There is the case of realizing a rediffraction optical system having a large numerical aperture NA with a high transmittance held by use of the limited number of lenses in the case of the wavelength in the range of the ultraviolet rays or of the far ultraviolet rays. In this case, the aberration cannot be sufficiently corrected and it is difficult to constitute the optical system by only the refracting materials.
FIG. 3 is a graph showing the spectral transmittance of the reducing projection optical system shown in FIG. 1. As will be obvious from this graph, when the use wavelength is below 300 nm, the transmittance is almost 0 and the wavelength in the range of the ultraviolet rays cannot be used.
On the other hand, since the example of FIG. 2 is constituted by only the reflecting mirrors, the use wavelength is not limited. However, since the main mirror 100 and auxiliary mirror 101 are coaxially arranged with respect to a point P on the axis of the object O and image I, the spherical aberration, coma aberration, and distortion aberration are preferably corrected. However, since the astigmatism in the tangential direction is large, the image plane is curved. To avoid this problem, a slit is used and an arc-shaped field is formed at such an image height that the astigmatisms in the tangential and sagittal directions coincide and the astigmatic difference becomes 0. For example, in the exposing of a mask pattern onto a semiconductive wafer, a necessary projection field is obtained by simultaneously scanning the mask as an object O and a wafer as an image plane I, thereby realizing the optical system near the zero aberration. However, when the reflecting optical system is constituted as a reducing optical system, it becomes as shown in FIG. 4. If the telecentric is held, all of the main mirror 100, a main mirror 102, and the auxiliary mirror 101 are not coaxially arranged. Namely, when the main mirror 102 and auxiliary mirror 101 are coaxially arranged around the point C, the center of the main mirror 100 is deviated to C', so that the outer axial aberration deteriorates. Further, since the object O and image I are not located on the same plane, it is necessary to individually scan the mask and wafer and to change the scanning speed in accordance with only the ratio of the image magnification. Since the error of the scanning speed results in the distortion of the image upon projection, it is necessary to accurately control the scanning speed. However, as the projected image becomes fine, it becomes fairly difficult to accurately control the scanning speed. Thus, in realizing the reducing optical system using the reflecting optical system as shown in FIG. 4, there are problems of the correction of the optical abberration and the mechanism.